1. Field of the Invention
The present invention relates in general to electronic circuits and in particular to clock generator circuitry.
2. Description of the Related Art
Sophisticated design and fabrication techniques are rapidly making practical systems-on-a-chip a reality. In turn, a broad range of personal and commercial hand-held appliances can be constructed which embody a high degree of functionality. These appliances include personal digital assistants, personal digital music players, compact computers, point of sale devices, and Internet access devices, to name only a few of the possibilities.
A number of factors must be addressed when designing a system-on-a-chip. Among other things, the device must be capable of interfacing with a broad range of input/output devices which may be required to support various potential user-defined applications. Moreover, the device must be power efficient while operating at high clock speeds. Additionally, should have a large address space to flexibly support a range of possible memory configurations and sizes.
In accordance with one embodiment of the principles of the present invention, clock generation circuitry is disclosed which includes an oscillator for generating a first signal from a crystal of a selected oscillating frequency. A first frequency multiplier selectively multiplies the frequency of the first signal by a predetermined factor to obtain a second signal having a frequency of a preselected multiple of a first set of clock signals. A divider selectively divides the frequency of the second signal by a second factor to obtain a third signal of a selected frequency.
A second frequency multiplier selectively multiplies the frequency of the third signal by a third factor to obtain a fourth signal of a selected frequency, the second and third factors selected to produce a fourth signal having a frequency of a preselected multiple of a second set of clock signals.
Among other things, the principles of the present invention allow for the construction and operation of a clock generator which can generate a number of clocks of widely varying frequencies from a single crystal. Moreover, the clock circuitry is easily programmable such that the crystals themselves are readily interchangeable. Clock generators embodying these principles are preferably utilized in integrated circuits, although these same principles may also be advantageously applied to clock generators constructed out of discrete devices.